Aspects of the present disclosure relate generally to cables, such as fiber optic cables that may support and carry optical fibers as well as other cable components. More specifically, aspects of the present disclosure relate to a binder film for constraining elements of a cable, such as buffer tubes wound around a central strength member in a core of a fiber optic cable.
Loose tube fiber optic cables typically use crisscrossing binder yarns that are counter-helically wrapped about a core of the cable to constrain stranded buffer tubes containing optical fibers, particularly with arrangements of the buffer tubes that include reverse-oscillatory winding patterns of the buffer tubes where the lay direction of the buffer tubes periodically reverses around a (straight) central strength member along the length of the core. The central strength member is typically a rod of a rigid material. Buffer tubes are typically cylindrical tubes (generally 2 to 3 mm in outer diameter) that contain optical fibers. Open space in the interior of a buffer tube may be water-blocked with grease.
Applicants have found that stranded buffer tubes, particularly those stranded in a reverse-oscillating pattern, function as a loaded dual-torsion spring with bias to unwind and correspondingly stretch out along the length of the cable. The binder yarns constrain the buffer tubes in the reversals. However, use of binder yarns may limit the length of cable that can be manufactured without stopping a manufacturing line. For example, due to finite lengths of binder yarns on a bobbin, the manufacturing line may be stopped every 20 kilometers (km) to switch out bobbins. Stopping the manufacturing line and switching out components reduces the efficiency. Further, binder yarns may impart distortions or stress concentrations in the stranded buffer tubes, where the binder yarns pass over the respective buffer tubes, potentially resulting in attenuation of optical fibers therein. The level of attenuation is a function of the tension in the binder yarns, which itself may be a function of the number, arrangement, structure, and materials of the buffer tubes, among other variables. Application of binder yarns may accordingly limit the speed of a stranding machine, depending upon allowable binder-yarn tension. A need exists for a binder system that allows for faster manufacturing of cables, reduces potential for attenuation of optical fibers in the cables (such as by avoiding point loading of buffer tubes), and/or allows for long, continuous lengths of such cables to be efficiently manufactured.
To this end, Applicants have experimented with manufacturing stranded cable cores without binder yarns. In one experiment, Applicants attempted to extrude a thin film over a core of stranded buffer tubes with binder yarns removed. The buffer tubes had previously conformed to the stranding pattern about the core and the pattern remained when the binder yarns were removed. However, a “bird nest” or jumble of stranded buffer tubes appeared upon extruding the thin film, which became more and more pronounced until the manufacturing line had to be stopped. Applicants theorize that the buffer tubes migrated axially forcing them outward and away from the central strength member when the binder yarns were removed. The thin film did not cool (and constrict) fast enough, with the stranded buffer tubes held down, to sufficiently couple the stranded buffer tubes to the central strength member of the cable. Instead, the buffer tubes shifted axially due to release of spring forces and pull of the extrusion cone, creating the “bird's nest.”
In another experiment, Applicants circumferentially taped only the reversal points of the stranded buffer tubes and to then extruded a jacket over the taped stranded buffer tubes. However, with this experiment a “bird nest” formed, resulting in bulges in the cable just prior to each reversal point of the stranded buffer tubes along the length of the cable. Applicants theorize that the stranded buffer tubes shifted axially between reversals. Release of spring forces in the stranded buffer tubes lifted the buffer tubes away from the central strength member. Axial loading (pulling) on the stranded elements by the extrusion cone then moved the buffer tubes axially, where excess length built up until coupling occurred with the tape. In view of the experimentation, a need exists for a binder system that overcomes some or all of the drawbacks associated with binder yarns, while limiting and/or controlling the impact of unwinding, outward- and axial-migration of the buffer tubes due spring forces in stranded buffer tubes and axial forces from extrusion.